I have a requirement for storing blank strings of length 1, 2, and 3 in some columns of my Hive table.
Storing:
If my column type is char, then I see that the data is always trimmed before storing. i.e. length(column) is always 0
If my column type is varchar then the data is not trimmed. so length(column) is 1, 2 and 3 respectively.
So that solves my storing problem.
Querying:
I am unable to query the column by value.
say. select * from hive table where column = ' ';
it only works if I do something like
select * from hive table where length(column) > 0 and trim(column) = '';
Is there a way to handle this separately ?
say I want to query those records where column value is of a blank string of length 3? How do I do this?
This is what i Tried (Note that the issues seems to be when the file is stored as parquet)
CREATE EXTERNAL TABLE IF NOT EXISTS DUMMY5 (
col1 varchar(3))
STORED AS PARQUET
LOCATION "/DUMMY5";
insert into DUMMY5 values (' '); // 2 character strings
insert into DUMMY5 values (' '); //3 character strings
select col1, length(col1) from DUMMY5;
+-------+------+--+
| col1 | _c1 |
+-------+------+--+
| | 3 |
| | 2 |
+-------+------+--+
select col1, length(col1) from DUMMY5 where col1 = ' '; // 0 record
select col1, length(col1) from DUMMY5 where col1 = ' '; // 0 record
Running Hive 2.1.1
drop table dummy_tbl;
CREATE TABLE dummy_tbl (
col1 char(1),
col2 varchar(1),
col3 char(3),
col4 varchar(3)) ;
insert into dummy_tbl values (' ', ' ', ' ', ' ');
select length(col1), length(col2), length(col3), length(col4) from dummy_tbl;
Result:
c0 c1 c2 c3
0 1 0 2
Varchar column works absolutely correct. col2 was trimmed on insert, it is documented.
col4 varchar(2) works correctly, this query returns 1:
select count(*) from dummy_tbl where col4=' '; --returns 1
And length of all char columns shows 0 and comparison ignoring spaces like it is documented:
select count(*) from dummy_tbl where col1=' '; --single space --returns 1
select count(*) from dummy_tbl where col1=' '; --two spaces --also returns 1 because it is ignoring spaces
You can use varchar with proper length. Or STRING type if you not sure about length.
I have this table where NULL is the NULL value, not the string NULL:
MYCOL
--------
NULL
example
Why does this query not return the NULL row?
select * from example_so where nvl(mycol, '') = '';
'' is again NULL in Oracle, because Oracle doesn't support empty Strings just like Other High Level languages or DBMS..
You need to look for NULL/empty string using IS NULL or IS NOT NULL
No other relational operator work against NULL, though it is syntactically valid. SQLFiddle Demo
It has to be,
select * from example_so where mycol IS NULL
EDIT: As per Docs
Oracle Database currently treats a character value with a length
of zero as null. However, this may not continue to be true in future
releases, and Oracle recommends that you do not treat empty strings
the same as nulls.
Because NULL = NULL is simply unknown. A third state perhaps? It is neither TRUE nor FALSE.
Oracle considers an EMPTY STRING as NULL.
nvl(mycol, '') makes no real sense, as you are making the NULL back to NULL and comparing it again with NULL.
SQL> WITH DATA AS(
2 SELECT 1 val, 'NULL' str FROM dual UNION ALL
3 SELECT 2, NULL str FROM dual UNION ALL
4 SELECT 3, '' str FROM dual UNION ALL
5 SELECT 4, 'some value' str FROM dual)
6 SELECT val, NVL(str, 'THIS IS NULL') FROM data WHERE str IS NULL
7 /
VAL NVL(STR,'THI
---------- ------------
2 THIS IS NULL
3 THIS IS NULL
SQL>
select * from example_so where nvl(mycol, '') = '';
nvl(mycol, '') will be resulted as NULL
and when you compared NULL with empty string it cant be compared
create table t(id varchar2(2));
insert into t values (nvl(null,'')); <------ this will insert NULL
insert into t values (nvl(null,'et'));
I was wondering about the possibility to count the null columns of row in SQL, I have a table Customer that has nullable values, simply I want a query that return an int of the number of null columns for certain row(certain customer).
This method assigns a 1 or 0 for null columns, and adds them all together. Hopefully you don't have too many nullable columns to add up here...
SELECT
((CASE WHEN col1 IS NULL THEN 1 ELSE 0 END)
+ (CASE WHEN col2 IS NULL THEN 1 ELSE 0 END)
+ (CASE WHEN col3 IS NULL THEN 1 ELSE 0 END)
...
...
+ (CASE WHEN col10 IS NULL THEN 1 ELSE 0 END)) AS sum_of_nulls
FROM table
WHERE Customer=some_cust_id
Note, you can also do this perhaps a little more syntactically cleanly with IF() if your RDBMS supports it.
SELECT
(IF(col1 IS NULL, 1, 0)
+ IF(col2 IS NULL, 1, 0)
+ IF(col3 IS NULL, 1, 0)
...
...
+ IF(col10 IS NULL, 1, 0)) AS sum_of_nulls
FROM table
WHERE Customer=some_cust_id
I tested this pattern against a table and it appears to work properly.
My answer builds on Michael Berkowski's answer, but to avoid having to type out hundreds of column names, what I did was this:
Step 1: Get a list of all of the columns in your table
SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'myTable';
Step 2: Paste the list in Notepad++ (any editor that supports regular expression replacement will work). Then use this replacement pattern
Search:
^(.*)$
Replace:
\(CASE WHEN \1 IS NULL THEN 1 ELSE 0 END\) +
Step 3: Prepend SELECT identityColumnName, and change the very last + to AS NullCount FROM myTable and optionally add an ORDER BY...
SELECT
identityColumnName,
(CASE WHEN column001 IS NULL THEN 1 ELSE 0 END) +
-- ...
(CASE WHEN column200 IS NULL THEN 1 ELSE 0 END) AS NullCount
FROM
myTable
ORDER BY
NullCount DESC
For ORACLE-DBMS only.
You can use the NVL2 function:
NVL2( string1, value_if_not_null, value_if_null )
Here is a select with a similiar approach as Michael Berkowski suggested:
SELECT (NVL2(col1, 0, 1)
+ NVL2(col2, 0, 1)
+ NVL2(col3, 0, 1)
...
...
+ NVL2(col10, 0, 1)
) AS sum_of_nulls
FROM table
WHERE Customer=some_cust_id
A more generic approach would be to write a PL/SQL-block and use dynamic SQL. You have to build a SELECT string with the NVL2 method from above for every column in the all_tab_columns of a specific table.
Unfortunately, in a standard SQL statement you will have to enter each column you want to test, to test all programatically you could use T-SQL. A word of warning though, ensure you are working with genuine NULLS, you can have blank stored values that the database will not recognise as a true NULL (I know this sounds strange).
You can avoid this by capturing the blank values and the NULLS in a statement like this:
CASE WHEN col1 & '' = '' THEN 1 ELSE 0 END
Or in some databases such as Oracle (not sure if there are any others) you would use:
CASE WHEN col1 || '' = '' THEN 1 ELSE 0 END
You don't state RDBMS. For SQL Server 2008...
SELECT CustomerId,
(SELECT COUNT(*) - COUNT(C)
FROM (VALUES(CAST(Col1 AS SQL_VARIANT)),
(Col2),
/*....*/
(Col9),
(Col10)) T(C)) AS NumberOfNulls
FROM Customer
Depending on what you want to do, and if you ignore mavens, and if you use SQL Server 2012, you could to it another way. .
The total number of candidate columns ("slots") must be known.
1. Select all the known "slots" column by column (they're known).
2. Unpivot that result to get a
table with one row per original column. This works because the null columns don't
unpivot, and you know all the column names.
3. Count(*) the result to get the number of non-nulls;
subtract from that to get your answer.
Like this, for 4 "seats" in a car
select 'empty seats' = 4 - count(*)
from
(
select carId, seat1,seat2,seat3,seat4 from cars where carId = #carId
) carSpec
unpivot (FieldValue FOR seat in ([seat1],[seat2],[seat3],[seat4])) AS results
This is useful if you may need to do more later than just count the number of non-null columns, as it gives you a way to manipulate the columns as a set too.
This will give you the number of columns which are not null. you can apply this appropriately
SELECT ISNULL(COUNT(col1),'') + ISNULL(COUNT(col2),'') +ISNULL(COUNT(col3),'')
FROM TABLENAME
WHERE ID=1
The below script gives you the NULL value count within a row i.e. how many columns do not have values.
{SELECT
*,
(SELECT COUNT(*)
FROM (VALUES (Tab.Col1)
,(Tab.Col2)
,(Tab.Col3)
,(Tab.Col4)) InnerTab(Col)
WHERE Col IS NULL) NullColumnCount
FROM (VALUES(1,2,3,4)
,(NULL,2,NULL,4)
,(1,NULL,NULL,NULL)) Tab(Col1,Col2,Col3,Col4) }
Just to demonstrate I am using an inline table in my example.
Try to cast or convert all column values to a common type it will help you to compare the column of different type.
I haven't tested it yet, but I'd try to do it using a PL\SQL function
CREATE OR REPLACE TYPE ANYARRAY AS TABLE OF ANYDATA
;
CREATE OR REPLACE Function COUNT_NULL
( ARR IN ANYARRAY )
RETURN number
IS
cnumber number ;
BEGIN
for i in 1 .. ARR.count loop
if ARR(i).column_value is null then
cnumber := cnumber + 1;
end if;
end loop;
RETURN cnumber;
EXCEPTION
WHEN OTHERS THEN
raise_application_error
(-20001,'An error was encountered - '
||SQLCODE||' -ERROR- '||SQLERRM);
END
;
Then use it in a select query like this
CREATE TABLE TEST (A NUMBER, B NUMBER, C NUMBER);
INSERT INTO TEST (NULL,NULL,NULL);
INSERT INTO TEST (1 ,NULL,NULL);
INSERT INTO TEST (1 ,2 ,NULL);
INSERT INTO TEST (1 ,2 ,3 );
SELECT ROWNUM,COUNT_NULL(A,B,C) AS NULL_COUNT FROM TEST;
Expected output
ROWNUM | NULL_COUNT
-------+-----------
1 | 3
2 | 2
3 | 1
4 | 0
This is how i tried
CREATE TABLE #temptablelocal (id int NOT NULL, column1 varchar(10) NULL, column2 varchar(10) NULL, column3 varchar(10) NULL, column4 varchar(10) NULL, column5 varchar(10) NULL, column6 varchar(10) NULL);
INSERT INTO #temptablelocal
VALUES (1,
NULL,
'a',
NULL,
'b',
NULL,
'c')
SELECT *
FROM #temptablelocal
WHERE id =1
SELECT count(1) countnull
FROM
(SELECT a.ID,
b.column_title,
column_val = CASE b.column_title
WHEN 'column1' THEN a.column1
WHEN 'column2' THEN a.column2
WHEN 'column3' THEN a.column3
WHEN 'column4' THEN a.column4
WHEN 'column5' THEN a.column5
WHEN 'column6' THEN a.column6
END
FROM
( SELECT id,
column1,
column2,
column3,
column4,
column5,
column6
FROM #temptablelocal
WHERE id =1 ) a
CROSS JOIN
( SELECT 'column1'
UNION ALL SELECT 'column2'
UNION ALL SELECT 'column3'
UNION ALL SELECT 'column4'
UNION ALL SELECT 'column5'
UNION ALL SELECT 'column6' ) b (column_title) ) AS pop WHERE column_val IS NULL
DROP TABLE #temptablelocal
Similary, but dynamically:
drop table if exists myschema.table_with_nulls;
create table myschema.table_with_nulls as
select
n1::integer,
n2::integer,
n3::integer,
n4::integer,
c1::character varying,
c2::character varying,
c3::character varying,
c4::character varying
from
(
values
(1,2,3,4,'a','b','c','d'),
(1,2,3,null,'a','b','c',null),
(1,2,null,null,'a','b',null,null),
(1,null,null,null,'a',null,null,null)
) as test_records(n1, n2, n3, n4, c1, c2, c3, c4);
drop function if exists myschema.count_nulls(varchar,varchar);
create function myschema.count_nulls(schemaname varchar, tablename varchar) returns void as
$BODY$
declare
calc varchar;
sqlstring varchar;
begin
select
array_to_string(array_agg('(' || trim(column_name) || ' is null)::integer'),' + ')
into
calc
from
information_schema.columns
where
table_schema in ('myschema')
and table_name in ('table_with_nulls');
sqlstring = 'create temp view count_nulls as select *, ' || calc || '::integer as count_nulls from myschema.table_with_nulls';
execute sqlstring;
return;
end;
$BODY$ LANGUAGE plpgsql STRICT;
select * from myschema.count_nulls('myschema'::varchar,'table_with_nulls'::varchar);
select
*
from
count_nulls;
Though I see that I didn't finish parametising the function.
My answer builds on Drew Chapin's answer, but with changes to get the result using a single script:
use <add_database_here>;
Declare #val Varchar(MAX);
Select #val = COALESCE(#val + str, str) From
(SELECT
'(CASE WHEN '+COLUMN_NAME+' IS NULL THEN 1 ELSE 0 END) +' str
FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = '<add table name here>'
) t1 -- getting column names and adding the case when to replace NULLs for zeros or ones
Select #val = SUBSTRING(#val,1,LEN(#val) - 1) -- removing trailling add sign
Select #val = 'SELECT <add_identity_column_here>, ' + #val + ' AS NullCount FROM <add table name here>' -- adding the 'select' for the column identity, the 'alias' for the null count column, and the 'from'
EXEC (#val) --executing the resulting sql
With ORACLE:
Number_of_columns - json_value( json_array( comma separated list of columns ), '$.size()' ) from your_table
json_array will build an array with only the non null columns and the json_query expression will give you the size of the array
There isn't a straightforward way of doing so like there would be with counting rows. Basically, you have to enumerate all the columns that might be null in one expression.
So for a table with possibly null columns a, b, c, you could do this:
SELECT key_column, COALESCE(a,0) + COALESCE(b,0) + COALESCE(c,0) null_col_count
FROM my_table
Is it possible to select 2 columns in just one and combine them?
Example:
select something + somethingElse as onlyOneColumn from someTable
(SELECT column1 as column FROM table )
UNION
(SELECT column2 as column FROM table )
Yes, just like you did:
select something + somethingElse as onlyOneColumn from someTable
If you queried the database, you would have gotten the right answer.
What happens is you ask for an expression. A very simple expression is just a column name, a more complicated expression can have formulas etc in it.
Yes,
SELECT CONCAT(field1, field2) AS WHOLENAME FROM TABLE
WHERE ...
will result in data set like:
WHOLENAME
field1field2
None of the other answers worked for me but this did:
SELECT CONCAT(Cust_First, ' ', Cust_Last) AS CustName FROM customer
Yes it's possible, as long as the datatypes are compatible. If they aren't, use a CONVERT() or CAST()
SELECT firstname + ' ' + lastname AS name FROM customers
The + operator should do the trick just fine. Keep something in mind though, if one of the columns is null or does not have any value, it will give you a NULL result. Instead, combine + with the function COALESCE and you'll be set.
Here is an example:
SELECT COALESCE(column1,'') + COALESCE(column2,'') FROM table1.
For this example, if column1 is NULL, then the results of column2 will show up, instead of a simple NULL.
Hope this helps!
To complete the answer of #Pete Carter, I would add an "ALL" on the UNION (if you need to keep the duplicate entries).
(SELECT column1 as column FROM table )
UNION ALL
(SELECT column2 as column FROM table )
DROP TABLE IF EXISTS #9
CREATE TABLE #9
(
USER1 int
,USER2 int
)
INSERT INTO #9
VALUES(1, 2), (1, 3), (1, 4), (2, 3)
------------------------------------------------
(SELECT USER1 AS 'column' from #9)
UNION ALL
(SELECT USER2 AS 'column' from #9)
Would then return : Result
Yes, you can combine columns easily enough such as concatenating character data:
select col1 | col 2 as bothcols from tbl ...
or adding (for example) numeric data:
select col1 + col2 as bothcols from tbl ...
In both those cases, you end up with a single column bothcols, which contains the combined data. You may have to coerce the data type if the columns are not compatible.
if one of the column is number i have experienced the oracle will think '+' as sum operator instead concatenation.
eg:
select (id + name) as one from table 1; (id is numeric)
throws invalid number exception
in such case you can || operator which is concatenation.
select (id || name) as one from table 1;
Your syntax should work, maybe add a space between the colums like
SELECT something + ' ' + somethingElse as onlyOneColumn FROM someTable
I hope this answer helps:
SELECT (CAST(id AS NVARCHAR)+','+name) AS COMBINED_COLUMN FROM TABLENAME;
select column1 || ' ' || column2 as whole_name FROM tablename;
Here || is the concat operator used for concatenating them to single column and ('') inside || used for space between two columns.
SELECT firstname || ' ' || lastname FROM users;